The inherent low loss, wide instantaneous bandwidth, lower bulk and light-weight over large distances make optical fiber communications an attractive choice over coaxially based systems. Fiber optic links have been demonstrated to operate from RF frequencies into the millimeter wave range. They employ an externally modulated approach based on the use of high power laser light sources as signal carriers. The availability of high power, low noise laser sources has enabled the design and implementation of analog fiber optic links in the microwave range which have 117 dB/Hz.sup.2/3 spurious-free dynamic range without linearization.
An externally modulated system consists of a laser source, an external modulator, optical fiber, and at least one optical detector. A continuous wave laser is used to drive an optical wave modulator that imposes an RF information-bearing signal on the optical carrier lightwave. The typical modulator used is a Mach-Zehnder interferometer, an electro-optic device fabricated on a lithium-niobate (LiNbO.sub.3) substrate. An RF information-bearing signal is impressed on a lightwave carrier within the modulator, yielding an amplitude-modulated signal which is coupled into a single mode fiber for transmission on a communications link. The modulated signal may be converted back to RF by an optical detector, commonly a PIN diode detector. The operational dynamic range of the link depends upon the total noise power at the detector, the maximum RF signal that can be detected, and the intermodulation distortion generated by the modulator.
The amplitude modulated output of the Mach-Zehnder modulator comprises a large carrier signal component and information-bearing upper and lower sidebands. The majority of the signal power transmitted on the optical link is concentrated in the carrier component; that is, that part of the signal output which does not convey information. The information is contained in the sidebands, which comprise a relatively small part of the optical power transmitted on the link. The ratio of signal power contained in the information bearing sidebands to the power of the carrier is termed the modulation index. The typical modulation index of an externally modulated optical link is on the order of 2-5% without further attempts to enhance the dynamic range of the link. The large carrier component in the modulator output produces a high DC level in the optical detector, thereby reducing its sensitivity to the much lower magnitude information signals on the link. This limits the dynamic range of the optical link.
Increasing the RF drive level to the optical modulator can increase the signal power of the information sidebands in the amplitude modulated output. However, because an electro-optical modulator, like its RF counterparts, is a non-linear device, it generates second and third harmonics and third-order intermodulation products when operated in the non-linear region of its transfer function. Thus, to keep the modulator operating in its linear region requires limiting the modulation index to a relatively low figure (on the order of 2-3%).
It is the concentration of signal power in the carrier, which carries no information, that limits the dynamic range in the optical link. A reduction of the carrier power coupled to the optical transmission fiber would improve the dynamic range of the optical link. Increasing the signal power of the information-bearing sidebands would further enhance dynamic range. This would improve the apparent modulation index of the signal coupled to the optical transmission medium by boosting the ratio of signal power to carrier power in the signal.